Migrate concreteTowerAlgebraMap recursion to Fin.dfoldl

[dhsorens]

Context

CompPoly/Fields/Binary/Tower/Impl.lean includes a TODO near concreteTowerAlgebraMap:

migrate to Fin.dfoldl

Proposed work

• Refactor concreteTowerAlgebraMap to use Fin.dfoldl.
• Preserve behavior and external API.

Acceptance criteria

• No API changes are required at call sites.
• proofs and/or tests to show correctness